Depends on whether you've captured Heatran or not. If you have, then there's nothing left to do other than train.
If you haven't, then you can do so. Go to Stark mountain and make your way to the farthest room in order to oust Team Galactic. Once you do, talk to Buck back in the survival area and then head back to the room where you finally got rid of Team Galactic for good (which would be the "farthest room" I just mentioned) and Heatran will be there for you to battle and capture. If you fail and KO accidentally, simply face the Elite Four and Cynthia again and it will be available for capture again after beating them. This only works if you've never captured it before (in the same game cartridge. IF you have it from another game catridge, you can still obtain this game's version).
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i think it is called stark mountain and it is also in Pokemon platinum
there are two trainers in stark mountain both of them have a gardevoir
you can get a persain by going near stark mountain and they sometimes appear there
Buck lives in the Survival Area in Pokemon Platinum. Buck is part of a quest involving Stark Mountain and Heatran.
They basically disappear for the rest of the game